Method for transmitting additional optical signals in the s+band

ABSTRACT

The invention relates to the transmission of optical signals (C-B ein ,L-B ein ) in conventional transmission bands, especially C-bands and L-bands, and of additional optical signals (S + -B ein ) in the S + -band by means of an optical fiber (OF). At least one optical pump signal (ps 1 , ps 2 ) is coupled into the optical fiber (OF) for amplifying the additional optical signals (S + -B ein ) by means of the Raman effect, whereby said additional signals are transmitted in the S + -band. The wavelength of the at least one optical pump signal (ps 1 , ps 2 ) is situated in the wave range of 1,320 nm to 1,370 nm.

The invention relates to a method for transmitting optical signals inconventional transmission bands and additional optical signals in the S⁺band via an optical fiber.

On account of the rapid growth of the Internet, it is possible toidentify a constantly increasing need for powerful transmission capacityin wide area technology, particularly in optical transmission systems.In this context, optical transmission systems, particularly transmissionsystems operating on the basis of the WDM principle (Wavelength DivisionMultiplexing), provide an opportunity for additionally increasing thedata capacity which can be transmitted via an optical fiber byincreasing the number of WDM channels used. At the present time, opticalsignals are predominantly transmitted in the C band (wavelength rangefor the WDM channels of approximately 1530-1560 nm), with first opticaltransmission systems of the L band (wavelength range for the WDMchannels of approximately 1570 to 1605 nm) having already beenannounced—conventional transmission bands. In addition, the developmentof the thulium-doped optical amplifier with a useful gain spectrum inthe S⁺ band (wavelength range for the WDM channels of approximately 1450to 1510 nm) has permitted additional transmission of further optical WDMchannels in the S⁺ band, as a result of which it is possible to producean additional increase in the optical transmission capacity.

In existing and future optical WDM transmission systems, optical signalsor transmission signals are transmitted primarily in the C band. In thecourse of extending already installed C-band transmission systems, itwould be advantageous, in particular, for an already existing C-bandtransmission system to be additionally used for transmission in the Land S⁺ bands. In this context, a fundamental criterion for designingsuch an optical transmission system is the optical path attenuation. Thespectral path attenuation minimum for the optical fiber or transmissionfiber, particularly a silicate glass fiber, (approximately 0.19 dB/km)is in the wavelength range around 1550 nm. Both in the C band and in theL band, the path attenuation for the WDM channel wavelengths furthestaway from the path attenuation minimum increases by approximately 0.01dB/km. By contrast, the sharp increase in the path attenuation profilein the optical fiber means that the shorter wavelengths in the S⁺ bandhave a path attenuation which is higher by approximately 0.1 dB/km, i.e.a path attenuation of approximately 0.29 dB/km, than the minimum fiberattenuation in the C band. This means that WDM channels in the S⁺ bandare subject to a much higher path attenuation than WDM channels in theconventional transmission bands—the C and L bands.

Furthermore, the publication “Trinal-wavelength band WDM transmissionover dispersion-shifted fibre” by J. Kani, et al., Electronic Letters,Feb. 18, 1999, vol. 35, No. 4 discloses that the stimulated Ramanscatter at high channel levels or total powers for the opticaltransmission signals can cause additional attenuation of the WDMchannels in the S⁺ band. In this context, the stimulated Raman effectproduced in the S⁺ band transfers energy from the shorter wavelengths—S⁺band—to longer wavelengths—C and L bands—and thereby increases thechannel levels of the WDM channels in the C and L bands at the cost ofthe WDM channels in the S⁺ band.

The relatively high path attenuation in the S⁺ band can, in principle,be equalized by relatively high channel levels at the optical fiberinput. In this context, however, the maximum channel level which can beproduced for transmitting the optical signals is limited by thenonlinear effects arising in the optical fiber, such as self phasemodulation or four wave mixing. However, when operating opticaltransmission systems in the C and L bands using such high channellevels, the channel levels of the transmission channels transmitted inthe S⁺ band cannot be increased to the required degree.

Another approach is to use lower channel data rates in the S⁺ band thanin the C and L bands. With comparable channel levels at the fiber input,the WDM channels in the S⁺ band have a relatively low opticalsignal-to-noise ratio (OSNR) on account of the relatively high fiberattenuation. The relatively low channel data rates in the S⁺ band meanthat the existing low signal-to-noise ratios in the S⁺ band can beevaluated using optical receivers for low data rates, however, whichmakes the relatively high path attenuation in the S⁺ band tolerable.

The object on which the invention is based is that of transmittingfurther WDM channels in an already existing optical WDM transmissionsystem in the S⁺ band without any great technical complexity, and, as afurther object, that of equalizing the increased fiber attenuation inthe S⁺ band as compared with the C and L bands. The object is achieved,on the basis of a method in accordance with the feature of thepre-characterizing part of patent claim 1, by the features of thecharacterizing part.

The fundamental aspect of the inventive method can be seen, inparticular, in that at least one optical pump signal is launched intothe optical fiber for the purpose of amplifying the additional opticalsignals transmitted in the S⁺ band on the basis of the Raman effect, thewavelength of the at least one optical pump signal being in thewavelength range from 1320 nm to 1370 nm. In a particularly advantageousmanner, this specifically equalizes the relatively high path attenuationin the S⁺ band using the stimulated Raman scatter in the opticaltransmission fiber, i.e. particularly the WDM channels in the S⁺ bandand the additional optical signals to be transmitted in the S⁺ band aresubject to an effective gain but the channels in the C and L bands areamplified only to a limited extent. The inventive method thus presents asimple technical method for upgrading an already existing optical WDMtransmission system for transmitting further WDM channels, particularlyin the S⁺ band, with little technical complexity.

Another fundamental advantage of the inventive method can be seen inthat the amplitude of the at least one optical pump signal is chosensuch that the fiber attenuation differences in the C band, L band and S⁺band of the optical fiber are virtually equalized—claim 2. The inventivechoice of the amplitude of the at least one optical pump signal makes itpossible, in a particularly advantageous manner, to compensate for therelatively high fiber dispersion in the S⁺ band until it correspondsapproximately to the fiber dispersion in the C band and in the L band ofthe optical fiber.

Another advantage of the inventive method can be seen in that at leasttwo optical pump signals having different wavelengths and/or lightoutputs are launched into the optical transmission medium for thepurpose of matching the levels of the amplified optical signals to aprescribed amplitude profile—claim 3. The inventive alignment of thepump wavelength and of the pump power for the optical pump signalsallows the levels of the additional optical signals transmitted in theS⁺ band to be amplified such that the levels of the additional opticalsignals have a prescribed amplitude profile after transmission in the S⁺band, which makes it possible to compensate for the path attenuationspectrum in the S⁺ band virtually completely.

In addition, the invention involves the at least one optical pump signalbeing launched into the optical transmission medium in the oppositedirection to the amplified additional optical signals—claim 4.Furthermore, the optical pump signal can be modulated with a data signalin order to produce a monitoring channel—claim 5, the modulationfrequency for this being chosen to be greater than 10 kHz. To reduce orto prevent disruptive overcoupling between the intensity noise of theoptical pump signals and the optical signals or transmission signals inthe frequency range used for transmission, the invention involves atleast one optical pump signal being launched into the optical fiber inthe opposite direction to the amplified additional optical signals, i.e.the S⁺ band in the optical transmission link is pumped in the oppositedirection. Such a contradirectional pump configuration results in alow-pass filter characteristic with a cutoff frequency of approximately10 kHz, i.e. the contradirectional pumping means that the optical pumpsignals have similar signal distortion to in the case of low-passfiltering using a low-pass filter which has a cutoff frequency ofapproximately 10 kHz. This effect can be utilized to produce furthertransmission channels by using a pump signal modulated significantlyabove the cutoff frequency of approximately 10 kHz to transmitadditional data or data signals.

The invention is explained in more detail below with reference to ablock diagram.

FIG. 1 shows, by way of example, a configuration for an optical pathsection OSA, with the optical path section OSA representing a WDMtransmission system part which can be used to implement datatransmission in the C, L and S⁺ bands, i.e. in the WDM channelsrepresented by the respective bands. The optical path section OSA shownin FIG. 1 has an optical transmission fiber OF with a first fiberconnection FA1 and a second fiber connection FA2. Furthermore, a firstto sixth bandpass filter BW1 to BW6 with a respective first, second andthird connection a1, a2, a3 is provided which can be used to launch oroutput optical transmission signals or signals OS. The connections a1 toa3 on the optical bandpass filters BW1 to BW6 are connected by means offirst to tenth optical connecting fibers OVF1 to OVF10. To produceoptical pump signals ps1,ps2, the optical path section OSA shown in FIG.1 is provided with a first optical pump unit PU1 and a second opticalpump unit PU2.

The first optical connecting fiber OVF1 is connected to the thirdconnection a3 of the first optical bandpass filter BW1, and the firstconnection a1 of the optical bandpass filter BW1 is routed to the thirdconnection a3 of the second optical bandpass filter BW2 via the secondoptical connecting fiber OVF2. Furthermore, the first connection a1 ofthe second optical bandpass filter BW2 is connected to the first fiberconnection FA1 of the optical fiber OF or transmission fiber OF via thethird optical connecting fiber OVF3. The second fiber connection FA2 ofthe optical fiber OF is routed to the third connection a3 of the thirdoptical bandpass filter BW3 via the fourth optical connecting fiberOVF4, and the second connection a2 of the third optical bandpass filterBW3 is connected to the third connection a3 of the sixth opticalbandpass filter BW6 via an eighth optical connecting fiber OVF8. Thefirst connection a1 of the third optical bandpass filter BW3 isconnected to the third connection a3 of the fourth optical bandpassfilter BW4 via the fifth optical connecting fiber OVF5, the firstconnection a1 of said third connection having the third connection a3 ofthe fifth optical bandpass filter BW5 connected to it via the sixthoptical connecting fiber OVF6. The first connection a1 of the fifthoptical bandpass filter BW5 is finally connected to the seventh opticalconnecting fiber. The sixth optical bandpass filter BW6 is connected tothe first and second optical pump units PU1, PU2, with, in particular,the first connection a1 of the sixth optical [lacuna] BW6 beingconnected to the signal output se of the first optical pump unit PU1,and the second output connection a2 of the sixth optical bandpass filterBW6 being connected to the signal output se of the second optical pumpunit PU2.

In the inventive optical WDM transmission system, an optical C-bandsignal C-B_(in), an optical L-band signal L-B_(in), and an opticalS⁺-band signal S⁺-B_(in) are thus transmitted, for example, the opticalC-band signal C-B_(in), the optical L-band signal L-B_(in) and theoptical S⁺-band signal S⁺-B_(in) each being able to contain a pluralityof WDM channels. The optical C-band signal C-B_(in) is launched into theseventh optical connecting fiber OVF7 at the start of the optical pathsection OSA shown in FIG. 1. The optical C-band signal C-B_(in) istransmitted from the seventh optical connecting fiber OVF7 via the fifthoptical bandpass filter BW5 and via the sixth optical connecting fiberOVF6 to the fourth optical bandpass filter BW4. The fourth opticalbandpass filter BW4 forwards the optical C-band signal C-B_(in) to thethird optical bandpass filter BW3 via the fifth optical connecting fiberOFV5, and the third optical bandpass filter outputs said optical C-bandsignal to the second fiber connection FA2 of the optical fiber OF viathe fourth optical connecting fiber OFV4.

Following transmission of the C-band signal C-B_(in) via the opticalfiber OF, the optical C-band signal C-B_(in), at the first fiberconnection FA1 of the optical fiber OF is output to the second opticalbandpass filter BW2 via the third optical connecting fiber OVF3. Thesecond optical bandpass filter transmits the optical C-band signalC-B_(in) to the first optical bandpass filter BW1 via the second opticalconnecting fiber OVF2 and finally to the end of the optical path sectionOSA shown in FIG. 1 via the first optical connecting fiber OVF1. Theoutput or end of the first optical connecting fiber OFV1 thus outputs anoptical C-band signal C-B_(out) which has been transmitted via theoptical path section OSA.

In addition to the optical C-band signal C-B_(in), an optical L-bandsignal L-B_(in) is transmitted which is launched into the sixth opticalconnecting fiber OVF6 via the fifth optical bandpass filter BW5, i.e.the L-band signal L-B_(in) is routed to the second connection a2 of thefifth optical bandpass filter BW5 and is thus launched into the sixthoptical connecting fiber OVF6 connected to the third connection a3 ofthe fifth optical bandpass filter BW5. The optical L-band signalL-B_(in) and the optical C-band signal C-B_(in) are transmitted via theoptical path section OSA in a first transmission direction UR1. Theoptical L-band signal L-B_(in) is thus transmitted to the fourth opticalbandpass filter BW4 via the sixth optical connecting fiber OVF6 andfinally to the third optical bandpass filter BW3 via the subsequentfifth optical connecting fiber OVF5. The third optical bandpass filterBW3 outputs the optical L-band signal L-B_(in) to the second fiberconnection FA2 of the optical fiber OF via the fourth optical connectingfiber OVF4, transmits it via the optical fiber OF and, at the firstfiber connection FA1 of the optical fiber OF, supplies it to the secondoptical bandpass filter BW2 via the third optical connecting fiber OVF3.The second optical connecting fiber OVF2 connected to the thirdconnection a3 of the second optical bandpass filter BW2 is used totransmit the optical L-band signal L-B_(in) to the first connection a1of the first optical bandpass filter BW1, where the optical L-bandsignal L-B_(in) is output and is passed to the second connection a2. Theoptical L-band signal L-B_(out) transmitted, in accordance with theinvention, via the optical path section OSA can consequently—not shownin FIG. 1—be supplied to a processing device for further processing.

In contrast to the optical L-band signal L-B_(in), the optical S⁺-bandsignal S⁺-B_(in) is transmitted in a second transmission direction UR2,running in the opposite direction to the first transmission directionUR1, i.e. the optical S⁺-band signal S⁺-B^(in) propagates in theopposite direction to the optical C-band signal C-B_(in) and in theopposite direction to the optical L-band signal L-B_(in) in the opticalfiber OF. The optical S⁺, band signal S⁺-B_(in) is launched via thesecond connection a2 of the second optical bandpass filter BW2 into thethird optical connecting fiber OVF3 connected to the first connection a1of the second optical bandpass filter BW2 and is routed to the firstfiber connection FA1 of the optical fiber OF. Following transmission ofthe optical S⁺-band signal S⁺-B_(in) via the optical fiber OF in thesecond transmission direction OR2, the optical S⁺-band signal S⁺-B_(in)is output at the second fiber connection FA2 and is supplied to thethird optical bandpass filter BW3 via the fourth optical connectingfiber OVF4. The third optical bandpass filter BW3 supplies the S⁺-bandsignal S⁺-B_(in) via the fifth optical connecting fiber OVFS to thethird connection a3 of the fourth optical bandpass filter BW4, where itis output and the second connection a2 outputs the transmitted opticalS⁺-band signal S⁺-B_(out). The optical S⁺-band signal S⁺-B_(out) whichis output using the fourth optical bandpass filter BW4 and istransmitted via the optical path section OSA can be supplied to afurther processing device for further processing—not shown in FIG. 1.

The pump units PU1, PU2 provided for the inventive amplification of theoptical WDM channels or the optical S⁺-band signals S⁺-B₁, transmittedin the S⁺ band are implemented as below in the exemplary embodimentshown. The first optical pump unit PU1 produces a first optical pumpsignal PS1 with a pump wavelength of 1340 nm, for example. The secondoptical pump unit PU2 forms a second optical pump signal PS2 with a pumpwavelength of 1360 nm, where the pump wavelength can inventively bechosen lie from the wavelength range of 1320 nm to 1370 nm. In thiscontext, the amplitude of the first and second optical pump signalsps1,ps2 is chosen such is that the fiber attenuation differences in theC band, L band and S⁺ band of the optical transmission fiber OF arevirtually equalized. In addition, the use of a first and a secondoptical pump unit PU1, PU2 shapes the Raman gain spectrum inventivelyforming in the S⁺ band such that the WDM channels in the optical S⁺ bandare amplified virtually evenly or the amplified optical S⁺, band signalsS⁺-B_(in), have a prescribed amplitude profile when they have beentransmitted via the optical fiber OF.

In the exemplary embodiment shown in FIG. 1, this is done, by way ofexample, by choosing a frequency difference between the first and thesecond pump signal PS1, PS2 of approximately 20 nm in order to produce aRaman gain spectrum which is matched to the S⁺-band signals S⁺-B_(in)which are to be amplified. The first pump signal ps1 is output by thefirst optical pump unit PU1 at the signal output se and is transmittedvia the tenth optical connecting fiber OVF10 to the first connection a1of the sixth optical bandpass filter BW6. In a similar manner, thesecond optical pump unit PU2 outputs the second optical pump signal ps2at the signal output se and routes it via the ninth optical connectingfiber OVF9 to the second connection a2 of the sixth optical bandpassfilter BW6. The sixth optical bandpass filter BW6 combines the first andsecond optical pump signals ps1, ps2 with one another and forwards themjointly to the third connection a3 of the sixth optical bandpass filterBW6 and transmits them via the eighth optical connecting fiber OVF8 tothe second connection a2 of the third optical bandpass filter BW3. Usingthe third optical bandpass filter BW3, the first and second optical pumpsignals ps1, ps2 are launched into the fourth optical connecting fiberOVF4 or into the optical fiber OF in the opposite direction to theoptical S⁺-band signal S⁺-B_(in), i.e. in the first transmissiondirection UR1. The optical path section OSA is thus pumped in oppositedirections using the first and second optical pump signals ps1, ps2,i.e. the levels of the WDM channels transmitted in the S⁺ band areincreased or amplified in the opposite direction to the propagationdirection using the first and second optical pump signals ps1, ps2, andhence the increased path attenuation in the S⁺ band for conventionalsilicate glass fibers is reduced.

Such a contradirectional pump configuration results in a low-pass filtercharacteristic with a coupling cutoff frequency of approximately 10 kHz,since the interaction length of the optical S⁺-band signal S⁺-B_(in)propagating in the opposite direction is passed through in a time periodof approximately 0.1 ms.

As a result of the contradirectional pumping of the optical transmissionfiber OF, the invention produces a Raman gain spectrum, forming in theS⁺ band, which effectively compensates for the path attenuation in theS⁺ band or amplifies the levels of the WDM channels transmitted in theS⁺ band in the optical fiber OF.

Furthermore, the low-pass filter characteristic, brought about by thecontradirectional pump configuration, with a coupling cutoff frequencyof approximately 10 kHz permits the first and/or second pump signal ps1,ps2 to be modulated with a data signal ds at a modulation frequency muchgreater than 10 kHz, which means that it will be possible to produce amonitoring channel for the optical path section OSA, for example. Tothis end, FIG. 1 contains a signal source SQ for producing a data signalds, said signal source being connected to a modulator M, for example aMach-Zehnder modulator, by means of a connecting line VL. The modulatorM is connected into the ninth optical connecting fiber OVF9 between thesecond pump unit PU2 and sixth optical bandpass filter BW6. The secondoptical pump signal ps2 output at the signal output se of the secondpump unit PU2 is transmitted to the modulator M and is modulated in themodulator M with the data signal ds produced by the data signal sourceSQ. The modulated second optical pump signal ps2′ is launched, in asimilar manner to the second optical pump signal ps2, into the opticalfiber OF via the sixth optical bandpass filter BW6 and the third opticalbandpass filter BW3. Following transmission via the optical fiber OF,the modulated second optical pump signal ps2′ is output using the secondoptical bandpass filter BW2 and is supplied, by way of example, to ademodulator DM connected to the second connection a2. The demodulator isused to demodulate the modulated second optical pump signal ps2′ and torecover the data signal ds′. The recovered data signal ds′ is suppliedto a monitoring device UE for the purpose of monitoring the optical pathsection OSA and is evaluated in the monitoring device UE.

In another implementation option, for the purpose of modulating one ofthe two optical pump signals ps1,ps2, the actuating signal for one ofthe two pump units PU1, PU2, particularly the laser unit containedtherein, can actually be modulated with the data signal ds, forexample—not shown in FIG. 1.

Furthermore, the inventive method is not limited exclusively totransmission of optical S⁺-band signals S⁺-B_(in) in addition to C-bandsignals C-B_(in) and L-band signals L-B_(in), but rather a WDMtransmission system can also be used, in accordance with the inventivemethod, for transmitting exclusively optical S⁺-band signals S⁺-B_(in).

The application of the inventive method is by no means limited to theWDM transmission system, but rather can be used for producing anyoptical transmission paths OSA.

1. A method for transmitting optical signals in conventionaltransmission bands and additional optical signals in an S⁺ band via anoptical fiber, the method comprising the steps of: launching at leastone optical pump signal into the optical fiber; amplifying, via the atleast one optical pump signal launched in the optical fiber, theadditional optical signals transmitted in the S⁺ band based on a Ramaneffect, with a wavelength of the at least one optical pump signal beingin a wavelength range from 1320 nm to 1370 nm; and choosing an amplitudeof the at least one optical pump signal such that fiber attenuationdifferences in a C band, an L band and the S⁺ band of the optical fiberare substantially equalized.
 2. A method for transmitting opticalsignals as claimed in claim 1, the method further comprising the step oflaunching at least two optical pump signals, having at least one ofdifferent wavelengths and different light outputs, into the opticalfiber for purposes of matching levels of the amplified additionaloptical signals to a prescribed amplitude profile.
 3. A method fortransmitting optical signals as claimed in claim 1, wherein the at leastone optical pump signal is launched into the optical fiber in anopposite direction to the amplified additional optical signals.
 4. Amethod for transmitting optical signals as claimed in claim 1, themethod further comprising the step of modulating the at least oneoptical pump signal with a data signal so as to produce a monitoringchannel.
 5. A method for optical signals as claimed in claim 4, whereina modulation frequency is greater than 10 kHz.